Google InDesign catalog plugin and you'll find tons of information about doing catalogs. For what you want it's a near certainty that you'll need a commercial plugin.
The general principle with master pages is they are where you put content that repeats on more than one page. Master page objects can be overridden and edited on the document page, but if you find a need to do that the odds are better than 90% that the object should not be on the master page to start with. When you say "text fields" do you really mean fields, or do you mean frames where you would enter text by typing? Fields implies some sort of linkage to data automation, and, no, I wouldn't expect to be able to do much editing on the field itself, but if you explain waht you mean, maybe we can figure out what's going on.
I do just mean text frames. What I had planned was to have the master page with all the frames in place for me to put in the title, desc., etc. and be able to alter the size of the frames if needed.
Since the pages are all going to use the same layout (four catalog items per page) I was hopeing the master pages whould be editable to save me doing a lot of copy & paste stuff with the frames.
I will look into the plugins to see what there is. Will develop my own one if need be, but thought indesign may have the ability built in.
InDesign does have a basic Data Merge function but it's quite primitive (really designed for things like mailing labels), and can do XML import, but waht you need to do to set up your files will vary quite a bit with what methods or plugins you use, so I'd suggest doing more research before you start in on template building.
To modify a text frame from the master page you would need to override it onto the document page by holding Cmd/Ctrl + Shift and clicking on it. but this would almost certainly be counter-productive in any sort of automation scheme.